Water-absorbing resin particles are used in a wide range of fields such as the use for sanitary materials such as sanitary products and disposable diapers, the use for agriculture and gardening such as water-retaining agents and agricultural ameliorants, or the use for industrial materials such as water-stopping agents and dew-condensation preventing agents. The water-absorbing resin particles are actively used especially in the fields related to the sanitary materials.
The water-absorbing resin particles used in the various types of use are lightly cross-linked high molecules and, for example, starch-based water-absorbing resins such as a hydrolysate of a starch-acrylonitrile graft copolymer (Patent Document 1) and a neutralized product of a starch-acrylic acid graft copolymer (Patent Document 2), a saponified product of a vinyl acetate-acrylic acid ester copolymer (Patent Document 3), and partially-neutralized products of polyacrylic acid (Patent Document 4, Patent Document 5, and Patent Document 6) are known.
These types of water-absorbing resin particles are each acquired by polymerizing, drying, and, when necessary, crushing and classifying while the water-absorbing resin particles are also modified after the polymerizing and the drying by adding various types of compound to the acquired water-absorbing resin particles to further impart additional functions thereto. A what-is-called surface cross-linking technique is known as a modification method for the water-absorbing resin particles according to which the vicinity of the surface of the water-absorbing resin particle precursor is cross-linked by a surface cross-linking agent. What is considered to be most important in the surface cross-linking treatment is to uniformly surface-cross-link the surface of each of the water-absorbing resin particles and, to do this, uniform mixing of the water-absorbing resin particle precursor and the surface cross-linking agent with each other before the surface cross-linking is important.
Such methods are known as the techniques to uniformly mix the water-absorbing resin particle precursor and the surface cross-linking agent with each other before the cross-linking is conducted, as a method of spraying a surface cross-linking agent in fine liquid droplets to thereby bring the surface cross-linking agent in contact with a water-absorbing resin powder (Patent Document 7), a method of spraying in an empty circular cone-shape showing an annular shape or in an elliptic cone-shape showing a double convex lens-shape (Patent Document 8), and a method of spraying a liquid onto water-absorbing polymer particles using a spray nozzle (Patent Document 9).